The present invention generally pertains to a distance measuring device and more particularly to a distance measuring device capable of emitting a distance measuring infrared light beam to a subject and receiving reflected infrared light from the subject to perform distance measurement.
In a conventional distance measuring device, an infrared light beam is projected toward a subject, and infrared light reflected from the subject is received by a light receiving device. Since the intensity and angle of incidence of the reflected light on the light receiving device is varied in accordance with the distance of the subject therefrom, the subject distance can be measured by determining the angle of incidence of light received by the receiving device.
A position sensitive device (PSD) is frequently used as a light receiving device. The PSD has two output terminals, and a current is generated at each output terminal depending on the intensity and angle of incidence of received light reflected from the subject. In a conventional PSD, the current output at one terminal is larger in magnitude when the subject is located at relatively large distances. The current output at the second terminal of the PSD becomes larger when the subject is located at relatively smaller distances from the light emitting unit. By calculating the ratio of the two generated currents or the corresponding two voltages, a signal which is dependent on only the angle of incidence of the reflected light can be obtained from the PSD.
A distance measuring device has been proposed in which an infrared light-emitting diode (IRED) is pulse driven to project an infrared light beam toward a subject. Infrared light reflected by the subject is detected by a PSD. The signal from the PSD is subjected to AC coupling to extract only a signal component, and the signal component thus obtained is suitably amplified. The amplified signal is integrated within the positive going pulses corresponding to the light emitting time, and the integration is repeated until the integration voltage reaches a predetermined voltage to thereby set the frequency of the integration in accordance with the intensity of the signal from the PSD.
The integration frequencies obtained until the two outputs from the PSD reach a predetermined voltage can be represented by Nf and Nn respectively. By calculating a value X in Equation 1, a stable distance measuring operation can be maintained irrespective of the reflectivity of the subject. EQU X=Nf/(Nf+Nn)(EQUATION 1)
Using the conventional method of calculating the value X from the integration frequencies Nf and Nn as described above there are numerous drawbacks, for example, if Nf and Nn are set at large values to improve distance measuring precision, count errors in the vicinity of a predetermined voltage become small and the distance measuring precision is improved. However, setting Nf and Nn to large values also results in the distance measuring time being lengthened. Conversely, if Nf and Nn are set at small values, while count errors in the vicinity of the predetermined voltage become larger, the distance measuring time is desirably shortened. However, while setting Nf and Nn to small values results in a shortened distance measurement time, it also results in the resolution being reduced and distance measuring precision being lowered.
Moreover, the signal subjected to the AC coupling described above is varied in intensity in accordance with the distance to the subject and the reflectivity of the subject. If the values Nf and Nn are set at large values, the precision of the value of X can be improved. However, in order to improve the precision of the calculated value of X, a variable-gain amplifier must be provided so that the device can detect small signals reflected by relatively distant subjects by using a large gain factor and can also detect large signals reflected from nearby subjects or subjects which are highly reflective using a small gain factor. However, the number of available gains which can be set in the amplifying circuit is increased and the size and complexity of the circuit is disadvantageously increased. There is therefore a need for a distance measuring device capable of precise distance measurement, yet small in size and able to perform distance measurement in a rapid time.